Beer-Lambert's law is the basis for optical quantitative analysis of matter concentration, and characterizes the absorption of light by a substance at a specific wavelength. As can be seen, in the case where the absorption cross section and the optical path length are known, the concentration of the corresponding gas can be calculated by measuring the incident light intensity and the exit light intensity.

I₀ (λ)---- incident light intensity of the light with a wavelength of λ;
I(λ)---- exit light intensity of the light with a wavelength of λ;
σ(λ)---- absorption cross section of the gas at the wavelength of λ;
C---- concentration of the gas to be measured;
L----action length of gas and light (effective length of gas pool);

SO₂ and NO have obvious characteristic absorption spectral lines in the range of 200-320 nm, so it is very suitable to simultaneously measure these two gases by ultraviolet spectrometry.

DOAS is the English abbreviation for differential optical absorption spectroscopy. The basic principle is to distinguish and invert the concentration of the gas to be measured by using the narrow-band absorption characteristics of the molecules of the substance to be measured.
The absorption of light by the sample gas can be divided into two parts: one part changes rapidly with the wavelength, and forms a narrow-band fine structure of the spectrum, which is only related to the characteristic absorption of the gas to be measured; the other part changes slowly with the wavelength, and forms a broad-band structure of the spectrum, which is further related to impurity scattering in the sample gas and other factors in addition to the characteristics of partial gas.
DOAS technique separates the narrow-band absorption portion of the absorption spectrum, thereby avoiding the effects of slow absorption of dust and other gases.

The optical path structure of the instrument is shown in the figure below: the ultraviolet light emitted by the ultraviolet light source enters the gas pool after being collimated by the collimating lens, and the ultraviolet light absorbed by the sample gas is focused by the focusing lens, coupled into the optical fiber and transmitted to the spectrometer. After grating splitting and CCD receiving optical signals for photoelectric conversion, the ultraviolet absorption spectrum of the gas to be measured is obtained. Finally, the concentration value of the gas to be measured is inverted by software algorithm.